Air traffic control system



oct. 28, 1952 J, YMAN ETAL 2,616,076

AIR TRAFFIC CONTROL SYSTEM Filed April 20, 1949 2 SHEETS-SHEET l DISTANCE MEASuR/NG ATTORNEY Oct. 28, 1952 J. I YMAN ETAL AIR TRAFFIC CONTROL SYSTEM 2 SHEETS-SHEET 2 Filed April 20, 1949 INVENTORS SEPH LY/V/N GEORGE B. ./TC/FORD J TTORNEY Patented Oct. 28, 1952 AIR TRAFFIC CONTROL SYSTEM Joseph Lyman and George B. Litchford, Huntington, N. Y., assignors to The Sperry Corporation, a corporation of Delaware Application April 2.0, 1949, Serial No. 88,522

10 Claims.

This invention relates to improvements in air traffic control systems and the like, and particularly to systems for producing a visual display showing the positions of aircraft or other mobile craft within a service area surrounding a reference station.

According to the principles of the invention, each aircraft or mobile station with which the system is to be used includes means for determining the distance and direction of the respective mobile station from the reference station. This information may be indicated or displayed at the respective mobile station and is in addition transmitted to the reference station. A device such as a facsimile recorder at the reference station is actuated by the signals from the various mobile stations to depict the respective positions thereof on a common visual display.

One of the principal objects of the present invention is to provide methods and means for conveying the above mentioned positional information from the mobile craft to the display device at the reference station.

Another important object is to achieve the foregoing object While substantially avoiding interference between the transmissions from two or more mobile stations.

A further object of the invention is to provide a system of the described type wherein the various mobile stations are interrogated in a sequence depending upon their respective directions from the reference station, and respond by reporting their respective distances.

The invention will be described with reference tothe accompanying drawings, wherein:

Fig. l is a schematic block diagram of a position determining and indicating system embodying the elements of the present invention, and

Fig. 2 is a schematic block diagram showing a system like that of Fig. 1 in more detail.

Referring to Fig. l, the reference station includes a timing signal generator I and a motor 3 which runs at a nominally constant speed. The generator I and the motor 3 are coupled together so that the rotation of the motor is synchronous with the signals from the generator. This may be accomplished either by having the motor 3 drive 4the generator i, or by making the output of the generator control the motor. The generator I produces a periodically recurrent signal such as a pulse or a wave which begins a cycle each time the shaft of the motor 3 passes through a certain angular position.

The output of the timing signal generator l is applied to a transmitter 5 and transmitted thereby to all the mobile stations with which the system is to operate, including the one shown at the right hand side of Fig. 1. Substantially any number of other mobile stationsyall similar to the one shown in Fig. l, may be used in cooperation with the reference station.

The mobile station includes a receiver 'I responsive to the transmitter 5 to reproduce the timing signals originated by the generator I. A motor 9 is controlled by the reproduced timing signals to drive its shaft I I in synchronism with that of the motor 3 at the reference station. An azimuth determining system I3 controls the angular position of a shaft I5 in accordance with the azimuth or direction of the mobile .station from the reference station. The terms azimuth'.

and direction as used herein are intended t'o mean the angle between a reference line, such as a meridian Vthrough the referencestation, and a line from the reference station to the mobileA station. Thus the azimuth is not related to the heading or direction of motion of the mobile station, but is simply the direction of the mobile station from the reference station.

The azimuth determining system responds to transmissions from the reference station, for example to signals transmitted by the transmitter 5, and it may include a radio direction finder and an earth compass arranged in known manner to rotate the shaft I5 according to azimuth. Alternatively, and preferably, the azimuth determining system may be of the omnidirectional radio range type, as Will be explained in connection with Fig. 2 of the drawings.

Radio distance measuring equipment (D. M. E.) is provided for determining the distance of the mobile station from thev reference station. This apparatus includes two units, a transmitterreceiver Il at the mobile station and a transponder it at the reference station. The transponder I9 comprises a receiver and a transmitter controlled by the output of the receiver to retransmit signals received from the mobile unit il. The mobile stati-on unit Il includes means for measuring the timing or phase of the transmitted signals with respect to the received return signals, and for rotating a shaft 20 accordingly. Since the delay of the received signals with respect to the transmitted-signals depends upon the time required fm` radiation to travel the round trip between the two stations, the position of the shaft 2G corresponds to the distance.

The shaft 2G is coupled to a variable frequency oscillator 2|. The frequency of the oscillator 2| is controlled by the shaft IS and thus depends upon and is a measure of the distance of the respective mobile station from the reference station. The output of the oscillator 2| is applied through a switch 23 to a transmitter 25. The switch 23 includes tWo relatively movable contact members 25 and 21, which may, for example, be rotatable about a common axis by the shafts and respectively. The contacts engageea'ch other only when the langular positions of the two shafts substantially correspond with each other.

A receiver 29 at the reference station is designed to respond to the mobile .station ltransmitter 25, as well as to the corresponding transmitters of any other similar'mobilesstations. The output of the receiver 29 is appliedto anindicator or display system 3|, which may include 'a polar facsimile recorder of thetypedescribed .in copending U. S. patent application SeriaLNumber 782,848, led October 29,1947, by JosephlLyman et al. and entitled Craft Position Plotting System.

xvThe recorder includes fan -arm 33 rex-tending radially l'from :a 1shaftl35 which is'fcouple'd to #the motorl3fanddriven thereby to sweepzthef-arm33 over l.ea..circ1ilar .area fon za .sheet '231 .of :sensitized 'facsimile :recording paper supported on .fa 'flat plate 39. The arm 33 carriesfiazpluralityof Styli "4| spaced. at small iinter-vals lzradially from ".-the shaft .35, "each connected ito -a respective :one 1 of a pluralitytof-:slipfrings 43'fonithe3shaft35. '.Each 'slip :ring fis .connected `through fa :brush to :an outputvterminal-ofxoneiofra ban-k '.of lters 45. I'he ltersr 45 arezdesignedxto pass-.differenti rela- .tively'snarrow frequency bands.' each .comprising alimited portion .of a :relatively wide continuous spectrum. L-Thednput terminals of thezltersf-,ll' are .connectedi toathe receiver .2.9.

In the op.eration .of 'the V:system -;cf .Fig. 1.1, the distance. measuringfequipment 1H, .19 :adjusts Lthe .frequency :of the .oscillator 2| :to fa -value `corre- .spending :to the distance -of :the mobile -.-station .from the-referencestatiOn. The f azimuthdetermining .system sets'the contactll of the :switch .23.-to-a .position corresponding tothe directionfor `azimuth of the mobile-.stationfrom ytheueference station. .The contact .25 iof the .switch 23 moves in..synchronism rwiththe arm-.33 of the-recorder at.the .reference station,so .that `whenever the angular .position .of the .arm V33 vcorresponds to the..az`imuth. of ,.the. mobile. station, the. switch Y23 closes.

Upon ,closure of the switch .23, the .output .of the 7variable frequency .oscillator .2| is .applied momentarilyto .the .transmitter l25 v.and Tis. reproduced at the reference.station.b5/-thereceiver.29. Thesignal is selectedby .one of the -.f1lters-45 'according lto its "frequency andis applied .to lthe 'oneof the styliM Whose distancefrom' the axisof the .arm "33 corresponds tothe distance of the mobile station Ifromthe .reference station. ".Thus.

energized momentarily, the stylus `produces I4a smallmark on thepaper31 at a .position corresponding to Ythatof the mobile station.

'During successive rotations of .the.arm-33.and the.sha'ft| |,.the` described operation is repeated. Iflthe mobile station isin motion,..the vsuccessive marks produced. on 4the paper .31 .will l-ie` oni aline representing .the track or-path vfollowed. bythe .mobile station. It Will -be :apparent-.that .other mobile stations. Aat :diferentazimuths will report their :distances -aty diiferent instants, l and :mobile .stations `at different fdistances vrwill f transmit signals of correspondingly idiierent frequencies. Thus .the number of ymobilexstations which .can be '.:dealt 'with substantially continuously :by ra 'single :reference :station iis :not limited `by the described system of position reporting and display.

Referring now .to Fig. 2, wherein parts similar to those of Fig. 1 are designated by corresponding reference characters, the azimuth determining system comprises an omnidirectional ratio range, wherein the reference station transmits a `rotating .directional pattern and a reference phase signal, and the mobile station fincludes means to compare the phase of the modulation produced by pattern rotation with that of the reference v.fp-hase signal to determine azimuth. The'omnidirectional range may be of any known *type l"'for-examr` le that described and shown in .copending U. 5. 4patent application Serial No. `r782,722, *Filed `October 29, 1947, by George B. Litchfordretfahrnow Patent No. 2,564,703 granted August 2l, 1951, and entitled Omni-Azimuth 'Guidance System. Although said system includes means for both coarse and fine determination of azimuth,'.:onlylthe: coarse portion will. :benescribed hereinbecause that isfallztl'iatxis;necessaryifcrza clear :understanding iof fthe lpresent invention.

A Xdirective :fantenna f5 designed to provide :a heartashapedfor'.limacon pattern, is rotatedxzontinuously at a nominally constant '-speedfof, for example, '3660 'R. 5P, Y:by -a motor 53. .The transmitteriis connectedto the gantennaiL A reference phase 'generator :55, -which :may be #a simple'alternator, f is @also driven by .the :motor-53 andgprovides-fan output of frequency equal tofthat of *the -f antenna lrotationy -i. fe. `3600 cycles per minute :or i60 'cycles persecond. -A subcarrier generator or= oscillator. 51provides .afsignalr of ,fsay kilocycles per second, vWhi'chiis =modulated.in axmodulator 59 byathe .output yof ythe reference yphase generator-.55. .Thefm-odulated subcarrier is applied to the :transmitter Y.5 to .modulate the .main y. carrier, vwhich z may f be-.of za superhigh .Ifrequency .such ...as .5000 .arnegacycles per second. It will `be understood .that V.the modulation .inqeach case maybe ,eitherr amplitudeorfrequency modulation.

.At .the..mobi1e station, `the .100 .ki1ocyc-le-subcarrier .in .the output .of fthe .receiver 11 -passes lthrougha .filter 16| 4to .a demodulator 163. .The output of the demodulator 63.inlu"des=a-6.0 cycle component .corresponding to the .outputof 'the reference .phase .generator 155, .and .this ,passes through. a ilter 65 .to.a,.phase meter 61.

`Owing .to .the .rotation .of f the antenna 5 l 'the .5.90.0 megacycle .carrier -received at 'Lthe .receiver .il varies in amplitude `60'timesper.;second. This `provides a6() cycle .component inthe .outputsof 'the receiver Whose phase depends upon .the azimuth of the mobile station. Thiscomponent, which may be called the directionalphasesignal, gcesthrough .ai-liter "69 to .theghase meter L61. The phasemetenmay be of .known'typ, rotating 4'theindicatorsha'ft l5 ina'ccordance with `variationsinthe phase relationship between' the two input signals. .'A 'dial'and Ypointer arrangement "||,'.calibrated in'terms iof azimuth, may'be'con- 'nected' to theshaft il 5.

'Thetiming signal is'providedin'the system "of Fig. `-2 as follows: Aphaseshifter '13 'is coupled to Yth'e'motor S'and ydriven therebyin synchronism withr the recordershaft'35. The rateoirotation lmay be -o'f vthe 'order often revolutions-.per minute. The output of Ithe phase "shifter "I3 :is fthus a 60 cycle Isignal `like fthe reference g phase fsignalf-butvarying continuously in phase'with -re'spect `thereto, Vat lthe 'rate *of 360 Idegrees per rotation of the recordersliaft 35. This signal`is :applied tofa modulator 75, where it =modulates a "subca'rrier generator I'by fa subcarrier generator 11. The frequency of this subcarrier may be 120 kilocycles per second, for example, and after modulation by the varying-phase timing signal it is applied to the transmitter 5.

At the mobile station, a filter 19 separates the 120 kilocycle subcarrier from the output of the receiver 7 and applies it to a demodulator 8| like the demodulator B5. A filter 83 passes the 60 cycle varying phase signal to a phase meter 85 which may be like the phase meter B1. The phase meter 95 drives the shaft Il. Since the phase relation between the two inputs to the phase meter 85 corresponds at every instant to the angular position of the recorder shaft 35, the shaft is rotated in synchronism with the shaft 35.

The mobile station portion I1 of the distance measuring equipment includes an oscillator 81 which modulates a transmitter B9. The oscillator 8'! is also connected to a phase detector 9|. The phase detector has a second input circuit which is connected to the output terminals of a receiver 99.

The output of the phase detector energizes or controls the energization of a motor 93 to drive the shaft 29. An indicator 95, calibrated in terms of distance, is coupled to the shaft 29. The shaft 29 is also coupled to the oscillator 81 so as to control its frequency.

The reference station portion I9 of the distance measuring equipment includes a receiver 91 designed to respond to the transmitter 89, and a transmitter 99 which is modulated by the output of the receiver 91 and is designed to operate at the saine frequency as the mobile station receiver 99.

In the operation of the distance measuring equipment, the output of the oscillator t1 is transmitted to the reference station and returned to the mobile station, by way of the transmitter 89, receiver 9`, transmitter 99, and receiver 99. The output of the receiver 99 is like that of the oscillator 81, but is delayed with respect thereto by the length of time required for radiation to make the round trip between the mobile station and the reference station.

The phase detector 9| will produce no output if the two inputs to it are 90 degrees out of phase. If they are less than 90 degrees out of phase, the phase detector 9| will energize the motor 93 to run in such direction as to increase the frequency of the oscillator 87. Conversely, if the two inputs are more than 90 degrees out of phase, the frequency of the oscillator 8'3 will be decreased. In either event, the frequncy is adjusted to a value such that the time for one quarter cycle (90 degrees) of the oscillator output is equal to the radiation travel time. Thus the angular position of the shaft 29 will depend upon the distance of the mobile station from the reference station.

The switch 23, instead of operating directly upon the connection between the oscillator 2| andthe transmitter 25 as in the system of Fig. 1, actuates a relay I9! to close momentarily contacts |03, |95 and lill'. The contacts |91 connect the oscillator 2| to the transmitter 25. The

contacts |95 connect the receiver 99 to the phase detector 9|, and the contacts |91 connect the oscillator 81 to the transmitter 89.

Another modification in the system of Fig. 2 is in the provision of a highly directive antenna |9g;for the receiver 29 at the reference station. This antenna is rotated by the motor 3 in synchronism with the recorder shaft 35, and always points, in the direction from which a transmisrom said reference station, including means at .said reference station for generating a periodic tive antenna sion may be received from a mobile station at any particular instant. The antennal09 may also be used for the reciver 91 of the distance measuring equipment I9, although a separate an- .tenna is indicated in Fig. 2 to avoid confusion.

The operation of the system of Fig. 2 is substantially the same as that of Fig. 1. The switch 23 at any particular mobile station closes momentarily each time the recorder arm 33 at the reference station passes through an angular position corresponding to the azimuth. This connects the distance reporting oscillator 2| to the transmitter 25; at this same time the direc- I|l9 at the reference station is pointed toward the mobile station.

It is possible and preferable under certain circumstances to design this antenna |09 to have such a narrow beam that the length of the time during which signals can be received from the mobile station transmitter 25 is determined by the antenna |99 rather than by the period of closure of the switch 23. The overall operation of the system will be the same except that the marks produced on the recording paper 31 may be somewhat smaller and more accurately posi- .tioned in azimuth.

The relay contacts and |01 prevent operation of the radio elements of the mobile distance measuring equipment except during the brief instants during each timing cycle when the switch 23 is closed. The distance information is nevertheless continuously available at the indicator 95, simply being corrected if necessary each time the switch 23 closes. Since each mobile transmitter 89 operates only at instants corresponding to the azimuth of the respective mobile station, no interference can occur as a result of two mobile stations at the same distance from the reference station transmitting signals of substantially equal frequency to the receiver 91. Also, since the receiver 9D is disconnected except when the transmitter 89 operates, signals transmitted from the reference station transmitter in response to signals from other mobile stations at different azimuths will not reach the phase detector 9 I.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made Without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A system for transmitting from a mobile station to a reference station information as -to the distance and direction of said mobile station timing signal and means for transmitting said timing signal to said mobile station, means at 'said mobile station for producing a distance Signah-varyingzaccording'ftozthe;distance'ofrsaidfmo- 'bile 'station from said Avreference :,station, and jmeans :periodically ztransmitting :said distance :signalszto -said .referencer station -zduringfintervals which :..are .delayed awith :respect to .fsaid timing 'signalby anfzamount corresponding-'tothedirec- ,tion fof said .mobile station 'from said l"reference station.

52. .A system1 OrtranSmittingTrom 'a mobile stavtionxto a reference station "information'as-1to .the "distance and direction of said mobile stationfrom -said reference station, including `means fat :said 'reference station for generating a .periodic-Etimzing'signal andfmeans f'for transmitting said timing signal to said: mobile fstation, means Lat y-said mobile station l'for zproducing fa Asignal 'having a vvariable "characteristic :and Vmeans for varying said characteristic according Vto i the :distance fof said l mobile v'station 'from said reference station, transmitter fmeans 'at "said *mobile vrstation, and means-"applying said 'distance characteristic -sig- 'nal-itofsaidmobile station transmitter =meansfsub y'stantially only -during'fbrief `intervals Which are delayed with respect to the beginning of each vperiodfo'i"said timing signal by an Iamount corresponding to the direction of said'mobile sta- 4tion frome said referencestation.

f3. =Inasys`tem of thedescribed type,a station latea referencelocation including a-source of pe lriodic timingsignals and means transmittingy said timing signals "to `atleast one mobile station, said llmobile fstation including Ycircuit closing means-responsive tosaidtiming' signal-and to the Adirection of `said mobile station vfrom said refer- 'ence-station to close momentarily nat an instant -fduringeach period #of :said V'timing signals which p -dependsuponfsaid direction, -means generating a 'signal `fhaving Ya variable characteristic and means yvarying KAsaid characteristic according 'to thed'istance of fsaid `mobile'station from' said `reference station, La transmitter at said'mobile sta- `tion yand means including -said circuit closing nmeans for applying sa-id distance-characteristic lsi'gn-altofsaid mobile station transmitter; Aa-re- 'cei-verflat said reference stationy responsive to'said mobile station transmitter to reproduce said 'dis- 'tance-characteristic signals, Aand vmeans vat fsaid reference station responsive to Jsaid reproduced distance-characteristic signals land their timing with respect to saidtiming signals'to indicate'the v"distance-and direction of said mobile station 'from A'sa-id; reference station.

'4.'In'1a system Vo'f-the described type, ya station 'at ialrei'erence location including a 'facsimile renoorden-s'canning-a recording medium periodically 'along-fa coordinate 'representing azimuth; a source 'of periodic timing signals synchronized with-said scanning and means transmitting said timing ssignalsftofat'least one mobilestation, said'mobile I-stationzincluding circuit yclosing means vresponisivezto said'timing signal and fto'the direction of -lsaidmofbile station'from: sai'd;referencezstation Vto close m'omentarily :at :'an instant Iduring each periodzof-said timing, signals which depends' upon said-direction, vmeans generating afsignal having a variable characteristic and means ,varying said characteristic according to .thedistance'of said rmobile :stationffrom said `reference station, a ltransmitterfat said mobilerstation and means in- :cluding/ssaidcircuit .closing means for applying said,distancefcharacteristic signal to said` mobile stationtransmitter; a receiver at said reference Istation :responsive to said mobile station transmitter -to reproduce `said distance-characteristic signals, and means ,at said ,reference station -or fio separating said Yreproduced :signals :according 'itc the distances ithey Arepresent tand :applying #said 'signals 'to respective `points on 4said re'cofding medium along a` coordinate representingdistanee `tofproducera visual recordofithe ,distance-'and 'direction of 'saidmobile station'fromzsaidzreference station.

15. In azsystem ofthe tzlescribedtype,` a-istation at a reference location including r.a;facsimile recorder scanninga recording medium periodically alongacircular coordinate representing azimuth, :a lsource of periodic timing Vsignals:synchronized vwith said scanning and means ftransniittingsad timing ysignals, to at least one :mobile station, said mobile station including circuit closing means responsive to said timing signal and to the direction fofsaid mobilestation 'from said-reference station to'close momentarilyat aninstantduring eachperiod of said timing signals which depends uponsaid direction, means generating afsignal having a yvariable frequency and means fvarying -'said frequency according to thedistanceoft-said mobile station from said reference station, fa transmitter atsaid mobile stationv and` means'including saidcircuit closing means for applying usaidvariable frequency signalto said mobile sta- 'tion transmitter; a receiver `at said reference-sta.- vtionresponsiveto saidmobile station transmitter to reproduce said variablefrequency'-signals;band pass filter means at said reference stationfor separating said reproduced signals vaccordingto the distances they vrepresent Aand applying rsaid signals torespective points "on psaid recording medium :along radial coordinates Yrepresenting distance toproduce Ya map-like visual record'of the distance and direction of said mobil'e;statio'n from said vreference station.

6. In `an air Atraffic control system, pthe combination at a mobile station of ,an omnidirectional radio range receiver providing two outputswhose vtime relationship depends upon vthe directionrf said mobile station from a reference pointymeans providing a signal characteristic ofthedistance of said mobile station'from saidreference'point; means providing a timing signal, Y.and means transmitting said distance-characteristic Vsignal ,only atinstants delayed with respect to saidltim- .ing signal by an amount which dependsuponthe time relationship between saidltwooutputsffrom said receiver.

7.',In an air .traffic control 'system, .the combination vat a mobile station .oranomnidirectionaliradio range receiver providing aconstarit phase reference signal `output and ra directional phase output whose phase relationship tosaid reference signal depends upon the directionof said mobile4 station from a reference point; means .providing a ysignal characteristic of the distance ofA said mobile station from said referencepoint; a transmitter, means providing a timing fsignal, and means 'periodically applying saiddistancecharacteristic signal-to said transmitter at .-instantsdelayed with respect to said-timing1signal :by an amount which depends l,upon ythezphase ,relationship between said reference signal and said directional signal.

,8. In an :air 'traffic .control system, "the .combination :at a VAmobile station of 1an Aomni-'directional radio rangereceiverfor'providing'twosgoutputs whose itime .relationship ,depends'upon the direction of said mobile station from La freference point; radio distance measuring equipment at said mobile station and 'auxiliary :equipment zat 'said reference point adapted :to cooperate therewith to measure 'the 4distance zo'fsaid; mobile station from said reference point; means providing a repetitive timing signal, andmeans re-y sponsive to said two outputs and to said timing signal to prevent operation of said distance measuring equipment at said mobile station except at repetitive instants delayed with respect to said timing signal by an amount corresponding to said direction. Q

9. The invention set forth in claim 8, including a distance indicator system at said mobile station, said distance indicator system including a servomotor, and means responsive to the repetitive output of said distance measuring equipment to energize said servomotor according to the difference between the actual position of said servomotor and the position corresponding to the distance measured by said distance measuring equipment, whereby said servomotor is maintained substantially continuously at ga position which corresponds substantially to said measured distance; and means responsive to the position of said servomotor for producing a signal cha-racteristic of said distance. g

1 0. The invention set forth in claim 9, further including a radio transmitter at said mobile stag5 REFERENCS CITED The following references are of record in the le of this patent: if

UNrrED s'rA'rEs PA'I'ENTS Number Name Date 2,252,083 Luck Aug. 12, 1941 2,421,106 Wight et ai May 27, 1947 2,467,202 Gardiner Apr. 12, 1949 2,480,123 Deloraine t al. Aug. 30, 1949 2,483,097 McIlwain Sept. 27, 1949 2,490,268 Herbst Dec. 6, 1949 2,513,282 Busignies July 4, 1950 2,517,540 Busignies Aug. 8, 1950 2,535,038 Busignies L Dec. 26, 1950 2,561,345 Deloraine July 24, 1951 

